This invention relates to the extrusion of honeycomb articles from plasticized ceramic-forming batch materials. More particularly, the invention relates to an improved method and apparatus for extruding honeycomb articles which include circumferential skins formed about a central cellular structure, and thick-skinned honeycomb articles produced thereby.
Skinned honeycomb extrusion of the prior art is accomplished by extruding plasticized ceramic-forming batch materials, such as cordierite ceramic-forming batch materials, through honeycomb extrusion dies to form structures having a central webbed cellular honeycomb structure surrounded by a thin integral outer skin layer. Such skins provide additional strength and a clean appearance to such honeycomb articles. Typically, the honeycomb extrusion dies employed to produce such skinned honeycomb articles are multi-component assemblies including, for example, a web-forming die body combined with a skin-forming mask. U.S. Pat. Nos. 4,349,329 and 4,298,328 exemplify die structures including skin-forming masks. The die body typically incorporates batch feedholes leading to, and intersecting with, an array of thin discharge slots formed in the die face, through which the batch material is extruded. This extrusion forms an interconnecting array of crisscrossing thin webs forming the central cellular honeycomb structure. The mask is generally a ring-like circumferential structure, typically in the form of a collar, defining the periphery of the skin of the honeycomb. The circumferential skin layer of the honeycomb article is formed by extruding the batch material between the mask and the die body.
Many of the known die constructions are designed specifically to overcome the problems of poor skin adherence to the webbed honeycomb core structure, and/or distortion of the peripheral webs of the core as the skin is joined therewith during extrusion. U.S. Pat. No. 4,349,329, for example, discloses an extrusion die particularly designed to minimize peripheral cell distortion. In that die and its operation, batch material supplied to form the skin is collected in a pooling zone 36 beneath the skin-forming die mask 1. This batch material is extruded through a skin-forming gap between the die body and mask to join with the extruded central cellular structure issuing from the die body. The central structure features thickened peripheral webs. These thickened webs resist distortion as the skin joins the central structure during extrusion.
U.S. Pat. No. 5,219,509 describes another die design wherein skin forming batch material also flows inwardly from a peripheral collection zone beneath the mask. In this design, however, skin batch flow is redirected by the mask and die body onto a flow path which is generally parallel with, with only a very slight convergence toward the honeycomb extrusion axis. This design also minimizes the distortion of web portions of the peripheral cells, in this case by limiting the lateral skin pressure applied to the peripheral cells.
U.S. Pat. No. 6,455,124 describes another design wherein batch material for the skin layer flows inwardly from a peripheral collection zone 30 beneath the mask 32 and is redirected by the mask and die body onto the skin forming gap 29. In this die design and method, the skin layer and web segments are provided with well-matched thermal expansions. In particular, I-ratios for the skin and webs are substantially the same. This is provided, in part, by extruding the skin a slower speed than the web segments which is thought to improve particle alignment.
The prior art also includes various means for controlling the thickness of the extruded skin. U.S. Pat. Nos. 4,668,176 and 4,710,123, for example, describe die designs wherein skin thickness can be controlled by controlling the width of the gap formed between the die body and mask. Also shown are means for adjusting the supply of batch material to the skin-forming region of the die.
Tightening emissions control regulations, particularly for automobiles, are requiring ceramic honeycomb designs with substantially decreased web thickness and increased channel density for improved catalytic efficiency. For example, the demand for thin-wall honeycombs, for example honeycombs having web thicknesses of 0.004 inches (0.10 mm) or less, is increasing substantially. At the same time, honeycombs incorporating greater number of cells, for example, greater than about 400 cells/in2 (about channels/cm2) are also in demand.
Although current extrusion die designs can be adapted to the extrusion of thin-walled honeycombs with no gross forming defects, certain new problems unique to these thin walled structures have been encountered. One significant problem is that such thin-walled structures cause lower strength in the fired ceramic article, which can lead to fractures and cracking during canning operations. Of course, one apparent way to combat the strength problem might be to provide a thicker skin to add strength. However, adding thicker skins causes additional problems. In particular, the thicker skins produced by conventional dies exhibit high internal thermal stresses that cause part failure due to thermal cycling. This occurs because of the difficulty in achieving matched coefficient of thermal expansion (CTE) between the webs and the thicker skins. The higher CTE of the skin is thought to be due to the poor degree of particle alignment in the skin achieved by current die designs. Thus, there is a need for a way of adding thicker skins to such thin-walled honeycombs, without also causing thermal stress and differential CTE problems.
As was described above, one way of combating the skin CTE issue was to extrude the skin at a slower rate than the web body, thereby attempting to achieve some level of improved preferential particle alignment in the skin. However, this may cause the skin to sometimes tear or otherwise causes the skin to separate from the webs (skin/web separation), especially in the case of thin-walled honeycombs.
Although adjustments to conventional extrusion methods and apparatus can produce defect-free fired honeycombs at conventional skin and web thicknesses, thin-walled honeycombs may suffer from extrusion defects, and in particular, skin/web separation and tears. Accordingly, there is a need for die designs which improve skin flow for thin-walled honeycombs, and also address skin defect problems.
Further, as discussed, conventional dies have had difficulty in extruding thick-skinned honeycombs having desired properties, largely because the CTE of the skins are much higher than the webs. In particular, the CTE difference is believed to be because of the relatively poor particle alignment within the skin. Thus, there is a need for an improved die design which may be utilized to form thick, well-aligned peripheral skins on honeycomb articles.
Further, conventional dies tend to wear rapidly and unevenly. Thus, extrusion lines need to be taken off-line after short runs to exchange and service the die. This results in significant undesirable down time of the extrusion lines and added manufacturing cost. Moreover, these dies tend to require many adjustments during the runs to account for uneven die wear in the skin forming regions of the die. Accordingly, die designs that are less prone to wear are also desirable.